In-vehicle network system

An in-vehicle network system includes a plurality of first controllers; a plurality of first communication lines, the first communication lines being respectively connected to the first controllers; a connector that connects an external device or a wireless communication device; a connection communication line that is connected to the connector; and a first relay device that relays between the first communication lines and the connection communication line. The first relay device is configured to determine whether a security level in data communication between the first controllers is increased according to a type of the external device or a communication state of the wireless communication device in a state in which the external device or the wireless communication device is connected to the connector, and the first relay device is configured to increase the security level when the first relay device determines that the security level is increased.